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Forces of AttractionGravity or gravitational forces happen when one object attracts another. We're not talking about finding someone really cute and adorable. We mean when the molecules of one object pull on the molecules of another object. It's like the Earth pulling on you and keeping you on the ground. That pull is gravity at work.Every object in the universe that has mass has a gravitational pull or a gravitational force. Even you have gravity, but when you compare your mass to the mass of the Earth, your gravitational pull isn't very impressive. When the Earth spins and gravity pulls on the clouds, weather can be affected. The Earth's gravity even holds the atmosphere close to our surface. Smaller planets that have low gravity or no magnetic field cannot maintain and atmosphere. 
Planetary GravityObviously, gravity is very important on Earth. The Earth's gravity keeps our planet orbiting the Sun, and the Sun's gravity pulls on us. The gravity of other planets also have an affect our orbit. The moon's gravity pulls on the Earth and makes the tides rise and fall every day. As the moon passes over the ocean, there is a swell in the sea level. As the moon passes to the other side of the planet, the swell occurs there. The tides are independent of the phase of the moon. The moon has the same amount of pull whether there is a full or new moon. It would still in the same basic place.We have to bring up an important idea now. The Earth always has the same pull on every object. If you drop an acorn or a piano, they will fall at the same speed. The Earth's gravity accelerates objects when they fall. It constantly pulls and objects constantly speed up. But what keeps the Moon from falling into the Earth if all of this gravity is so strong? The answer is that the moon is revolving around the Earth. Those revolutions (about once a month) create a force that makes the Moon want to spin off into space. The Earth's gravity holds it in orbit. Think about holding a ball on a string and spinning in a circle. If you were to cut that string, the ball would fly off. That string is like the Earth's gravity. 
They Always ask About FeathersPeople always say, "What about feathers? They fall so slowly." Obviously, there is air all around us. When a feather falls, it falls slowly because the air is in its way. There is a lot of air resistance and that resistance makes the feather move slower. The forces at work are the same. If you dropped a feather in a container with no air (a vacuum), it would drop as fast as a baseball.
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Go for site help or list of physics topics at the site map!©copyright 1997-2007 Andrew Rader Studios, All rights reserved. Current Page: Physics4Kids.com | Motion | Gravity |
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